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Date:   Mon, 18 Sep 2017 11:11:15 +0200
From:   Greg Kroah-Hartman <>
Cc:     Greg Kroah-Hartman <>,, Eric Dumazet <>,
        Dmitry Vyukov <>,
        Tom Herbert <>,
        "David S. Miller" <>
Subject: [PATCH 4.12 21/52] kcm: do not attach PF_KCM sockets to avoid deadlock

4.12-stable review patch.  If anyone has any objections, please let me know.


From: Eric Dumazet <>

[ Upstream commit 351050ecd6523374b370341cc29fe61e2201556b ]

syzkaller had no problem to trigger a deadlock, attaching a KCM socket
to another one (or itself). (original syzkaller report was a very
confusing lockdep splat during a sendmsg())

It seems KCM claims to only support TCP, but no enforcement is done,
so we might need to add additional checks.

Fixes: ab7ac4eb9832 ("kcm: Kernel Connection Multiplexor module")
Signed-off-by: Eric Dumazet <>
Reported-by: Dmitry Vyukov <>
Acked-by: Tom Herbert <>
Signed-off-by: David S. Miller <>
Signed-off-by: Greg Kroah-Hartman <>
 net/kcm/kcmsock.c |    4 ++++
 1 file changed, 4 insertions(+)

--- a/net/kcm/kcmsock.c
+++ b/net/kcm/kcmsock.c
@@ -1383,6 +1383,10 @@ static int kcm_attach(struct socket *soc
 	if (!csk)
 		return -EINVAL;
+	/* We must prevent loops or risk deadlock ! */
+	if (csk->sk_family == PF_KCM)
+		return -EOPNOTSUPP;
 	psock = kmem_cache_zalloc(kcm_psockp, GFP_KERNEL);
 	if (!psock)
 		return -ENOMEM;

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